Plasma processing devices, such as plasma CVD (chemical vapor deposition) devices, dry-etching devices or ashing devices, are generally used in the process of producing semiconductor devices, liquid crystal displays, solar cells or other devices. In these devices, generation of high-density plasma covering a large area with a high level of uniformity is required for various purposes, such as dealing with large substrates, increasing the processing rate, and creating fine patterns. A plasma generation technique that is generally used for those purposes is a parallel-plate system. The parallel-plate system uses a pair of plate electrodes disposed parallel to each other in a vacuum container and generates plasma between them by applying a radio-frequency voltage to one plate electrode, while the other electrode is either connected to ground or applied with another radio-frequency voltage. This system is a so-called “capacitive coupled plasma (CCP)” system, which has an advantage in that it has a simple structure and yet can create approximately uniform plasma over the surface of the electrodes. The system also allows the use of large plate electrodes, so that even a large substrate can be processed relatively easily.
In recent years, plate electrodes have been extremely large to cope with the rapid increase in the size of glass substrates for display devices, causing a non-negligible degree of wavelength effect of the applied radio-frequency voltage. As a result, the electron density (plasma density) and other properties in the plane of the electrode plates have become noticeable. As one method for overcoming the drawbacks of the CCP system using a radio-frequency voltage, an inductively coupled plasma (ICP) system, which uses a small-sized antenna and a radio-frequency voltage, has been developed. ICP is generally known as a technique capable of producing high-density plasma with a low electron temperature and low ion energy even at lower gas pressures than used in the CCP system.
Patent Document 1 discloses a technique relating to ICP. The device disclosed therein includes a vacuum container for processing a substrate and another vacuum container for generating plasma (this container is hereinafter referred to as the “plasma chamber”), the two containers being connected with each other. The plasma chamber is made of a dielectric material, such as quartz glass or a ceramic, and a coil having a number of turns equal to or greater than one is wound around the plasma chamber. When a radio-frequency current is passed through this coil, an electric field is inducted within the plasma chamber. The induced electric field ionizes gas molecules within the plasma chamber, thus generating plasma. This technique can be classified as a plasma generation method using an external antenna. However, it has a problem due to the use of the dielectric plasma chamber: The dielectric container needs to be made of quartz glass, alumina or similar materials, which are rather low in mechanical strength and hence easy to be broken and hard to handle. This unfavorable characteristic also impedes the creation of large devices.
Patent Document 2 discloses a plasma generation device with an internal antenna. This internal antenna system includes a loop antenna provided in a vacuum container, with one end of the antenna connected via a matching box to a radio-frequency power source and the other end connected to ground either directly or via a capacitor. Similar to the device disclosed in Patent Document 1, plasma is generated when a radio-frequency current is passed through the antenna. In order to uniformly generate plasma over a large area within the vacuum container, the antenna needs to be extended along the inner wall of the vacuum container. However, this makes the antenna to have a length that approximates to the inner circumferential length of the vacuum container, increasing its impedance to radio-frequency waves. When, in this case, the radio-frequency current varies, the potential difference between the two ends of the antenna significantly changes, which results in a significant change in the plasma potential.
To solve such a problem, a non-loop antenna system using a wire conductor is disclosed in Patent Document 3. Furthermore, a system using a small-sized non-loop antenna to achieve low impedance has also been proposed (Non-Patent Document 1). It has been reported that, by using these techniques, high-density plasma having a low electron temperature can be generated under relatively low gas pressures. Additionally, the technique of distributing a plurality of antennas over an area is disclosed as a method for processing a large-sized substrate by using small-sized non-loop antennas.
In the non-loop antenna system, the antenna conductor is shaped as a pipe, through which cooling water is circulated to suppress the rise in temperature of the antenna due to the radio-frequency current. In this system, it is necessary to provide, at both ends of the pipe, a vacuum seal, a connector for the cooling water, and a connector for electrical connection to the radio-frequency power source or the ground. Such a configuration is not only complex but also causes problems, for example, in attaching or removing the antenna and in its maintenance and inspection. Attempting to achieve even lower impedance requires a larger diameter of the pipe of the antenna conductor so as to increase the passage area for the radio-frequency current. This causes the antenna conductor to have a larger radius of curvature, which is unfavorable, for example, in that the antenna needs to be longer.
It is generally known that superposing a magnetic field in the vicinity of the antenna is effective for generating stable plasma under low gas pressure or increasing the plasma density under high gas pressure. However, in the aforementioned structure with the antenna sticking inside the vacuum container, it is difficult to effectively apply a magnetic field to the plasma generation area in the vicinity of the antenna.